Method of making hollow casting molds



Dec. 29, 1942.4 l E ZAHN 2,306,516

METHOD oF MAKING HOLLOW CASTING MOLDS Filed Sept. 27, 1941 2 Sheets-Sheet l ATToR EYS Dec. 29, 1942. E H, ZAHN 2,306,516

METHOD OF MAKING HOLLOW CASTING MOLDS Filed sept. 27, 1941 2 sheets-sheet 2 -1NVENT0R- Enc HZahn,

BYv Mvg/W TTOR/VE S,

l rimas o 29,1942

Eris n. zahn, smal rome.- N.

Austenal laboratories, Inc., New York,'N. Y;, a `corporation of New York Y., r to Application 'September 27,-1941, Serial No. 412,567v 4 Claims. v(Cl. 22-190) My present invention relates generally to casting, and has. particular reference to improvements in the art of making hollow casting molds.

The invention is directed to the creation of the .well-known type of hollow mold which is sectional in character, whose casting cavity has substantially the same configuration and volumetricV proportions as a predetermined pattern, and Whose inner surface conforms to the convolutions of said pattern, the mold being intended for use in the creation of an article to be cast therein. The particular use to which the mold is put is not of prime consideration, but my invention is particularly useful in theproduction of master molds, composed preferably of metal, adapted to be used repeatedly for the creation of volatile or meltable cast articles intended themselves to be employed as investment patterns in the creation of heat-resistant casting molds of plaster or the like.

By means of the present invention, I am enabled to make a sectional hollow mold whose inner surface conforms with unusual accuracy to the ccnvolutional details of the original pattern. Such metallic articles as screws or other threaded elements, or articles having similar relatively fine or sharp convolutions, edges, or recesses, maybe used as original patterns in the practice of my invention without appreciable loss of detail, fineness, or accuracy during the course of transferring such convolutions to the inner surface of the resultant mold. As a consequence my' invention permits the quantity manufacture, by casting procedures, of many different types of articles whose nature or configuration has made it unfeaslble heretofore to produce them in the form of castings. Where'the material of which they are.to be made is unusually hard or tough, as in the case of many alloys of heat-resistant character, the present procedure becomes of particular value, because it avoids the necessity for forgswaging, or machining. Thus, my invention makes it possible to manufacture a large-variety of different articles out of certain special castable alloys which, by virtue of extreme hardness, toughness, or other qualities, are unsusceptible, from a practical standpoint, to machining, swaging, or similar treatment.,

My improved procedure is predicated upon the use, as alparting medium, of a freely stretchable membrane of inappreciable thickness, for example, a rubber sheethaving a normal thickness of approximately one thousandth (.001) of an inch or less.l Such a sheet, when stretched, reduces in thicknessto an even greater extent. often asbeingascrew,

-sumlng in practice an inappreciable thickness of only two ten-thousandths (.0002) of an inch or even less. v

In accordance with my invention, such a membrane is applied to a selected portion of the original pattern, and a mold section is then cast against the membrane-covered portion of the pattern. The advantage that is achieved results from the fact that the space between the mold section and pattern is at no place of absolute zero thickness nor preciable normal thickness of the membrane.

By way of contrast, it is pointed out that no ordinary parting medium, whether liquid or solid, produces any such desirable result, vbecause it inevitably forms accumulations of undesirable thickness in the valleys of the pattern, (thereby transferring an inaccurate -dulled contour to the mold section), and because it simultaneously tends to wearaway completely 'from the peaks of the pattern (thereby failing to fulfill its function as a parting medium). The present membrane, on lthe other hand, is operative as a parting medium throughout its extent, regardless of how much it may be stretched, and even the finest and sharpest convolutional details of the pattern are always accurately and faithfully transferred to the mold section that is being formed.

The present membrane has another advantage,

since its use as a parting medium makes it feasible lto produce a mold section having exactly parallel surface portions or similar contours which would ordinarily require a taper or draft to permit separation of the cast mold section from the pattern. The yieldable nature of the present membrane, even when stretched, facilitates such separation and makes it unnecessary to impart any special taper to the pattern for this purpose.

In some instances, where the pattern has a recess, it may be desirable as a, preliminary step, independently to form a mold portion adapted to fit, snugly into said recess, and to position such mold' portion in the recess to form an augmented pattern. The independently-formed mold pory tion is ultimately disassociated from the pattern and positioned intol corresponding association with the complementary mold sections that have been produced.

In the accompanying drawings-I have'chosen to illustrate my invention as it may be employed in the manufacture of two typical articles, one the other an arbitrarily-shaped greater than the inap-` thickness lof piece'having a relatively deep recess therein. In the drawings:

Figure l is a perspective view of an illustrative original pattern;

Figure 2 is a plan view of the pattern initially mounted on a suitable support;

Figure 3 is a cross-sectional view taken substantially along the line 3 3 of Figure 2, showing the present special membrane in position;

- Figure 4 is a view similar to Figure 3 showing the step of forming one of the mold sections;

Figure 5 is a similar cross-sectional view, illustrating the manner in which another of the mold sections is produced;

Figure 6 is a similar cross-sectional view showing the resultant hollow mold;

Figure 7 is an exploded view of another illustrative original pattern and a special mold porl tion for association therewith; and

Figures 8 to 12 inclusive are views similar to and corresponding to Figures 2 to 6, respectively, showing the manner in which the pattern of Figure 7 is employed.

The pattern 20 which I have shown in Figure 1 may be an actual specimen of the cast metal article ultimately to be manufactured in quantity. The particular item I have chosen to illustrate is an elongated element having a screwthreaded portion 2| and a head 22 with a transverse slit 23 therein, and is an example of the type of screw that is currently used in bone surgery.

The rst step in carrying out the present procedure is to mount this pattern in a suitable `manne`r.

This is preferably accomplished by embedding it in a block of plaster 24, or in any other suitable mounting material, the pattern being embedded only to a proper selected extent, depending upon its configuration. In the case of a screw of the character shown, it is preferably embedded substantially half-way so that only the upper half of its surface is exposed. In the case of a screw, it is also preferable to associate with it a thin strip of metal 25 which is snugly fitted into the slit 23, this metal strip being also partially embedded in the plaster as shown most clearly in Figure 3.

The plaster block 24 may also be provided with one or more suitable depressions 26 adapted to form a positioning lug or lugs on the mold sections that are to be produced.

After the pattern has been embedded in proper manner, the next step consists in applying to its exposed surface a special membrane 21. This membrane is of freely stretchable material and is purposely chosen to have an inappreciable thickness that is as minute as possible. I have found that.l a rubber sheet having a normal approximately one-thousandth (.001) of an inch, or even less, is suitable for the present purpose. This sheet or membrane is marginally held in position in any suitable manner, and I have not deemed it necessary to illustrate any particular kind of holding means.

A mold section 28 is then formed by casting the metal of the mold section against the membrane-covered surfaces. The mold section may be composed of any suitable material, and I have found it satisfactory, in practice, to employ an alloy of low melting-point, such as a tin-bismuth alloy composed of approximately sixty parts of tin and forty parts of bismuth. Such an alloy has a melting point approximately between 300 and 400 F. 'I'his temperature is insufcientto have any deleterious effect upon a rubtemperature that would injure or impair thev particular material of which the membrane may be composed.

The process of casting the mold section 28 is carried out in a suitable flask or other support (not shown) so as to impart a convenient contour to the side walls. Generally, a substantially rectangular flask is satisfactory.

The casting of the mold section 28 is effected under pressure. By the term pressure I intend to refer to a force which is sufficient to stretch the membrane firmly against the pattern and into accurate conformity with the surface configurations of the pattern. While this pressure of the membrane against the pattern may be produced in various ways, and while the pressure may be varied to suit differing requirements, I have found that a molten tin-bismuth alloy of the character hereinbefore mentioned may be satisfactorily cast under a pressure of approximately eighty pounds per square inch. Under certain circumstances the weight of the metal itself may be sufficient to produce the required pressure Casting of the mold section brings about a.

stretching of the membrane 21 so that it conforms accurately to every elevation and depression of the surface beneath it. The stretching of the membrane may reduce its thickness to as little as two ten-thousandths (.0002) of an inch, or even less, yet in n0 place is the thickness ever reduced to absolute zero. Accordingly, the contours transferred to the inner face of the lmold section 28 are an accurate negative duplication of the contours of the original pattern. For example, the sharp attenuated peaks of the screwthreaded portion 2| of the pattern produce correspondingly ne depressions in the mold section 28; and even the most delicate depressions in the pattern produce accurately-conforming elevated portions in the mold section. This desirable result is achieved without any danger of having the material of the mold section adhere to the pattern, since the membrane serves as a continuous unbroken parting medium throughout its entire extent.

After the mold section has been completed, the parts are separated and the plaster block 24 may be discarded. The section 28 i's then used as a support for the pattern 20, the latter being laid directly into the section 28, as shown in Figure 5, with no intermediate membrane.

To facilitate an understanding of the relationship between Figures 4 and 5, the mold section 28'has been' designatedin each of these figures as Mold section A. In producing the complementary Mold section B, the membrane 21 is again laid over the pattern and the exposed surface of the section 28, and the mold section 29 is thereupon cast against the membrane-covered surfaces. This casting is again accomplished under suitable pressure of the membrane against the pattern and a similar stretching of the membrane 21 takes place.

It is understood, of course, that the pattern surface over which the membrane 21- is applied in Figure 5 is the surface that was previously embedded in the plaster 24. That is, the procedure illustrated in Figures 2-5 exemplifies one ing medium successively to selected diiferentportions of the pattern, and successively casting complementary mold sections against the membrane-covered portions of the pattern.

The resultant mold. is shown in Figure 6. It is sectional and hollow and is adapted to be used in the creation of an article to. becast therein. With this object in view, a suitable sprue or sprues are formed, either during the production of the mold sections or after they have been formed. I, have illustratively shownin Figure 6 a transverse sprue 30 vcommunicating with a portion.3l which lies along the inner surface of the vmold section 29 and communicates with one end of the mold cavity. As herelnbefore stated, the particular use to which the mold'is put is optional, but the present invention is particularly. `useful in repeatedly using. the mold of Figure 6 as a master injection mold in which wax patterns are cast. Each of these patterns, when produced, -has the same shape as that illustrated in Figure 1, and is itself intended to be used as an investment pattern in the creation of a subsequent heat-resistant casting mold of plaster or the'like. It is the last-mentioned mold which is ultimately used for the purpose of producing a metal casting of the character shown in Figure 1.

While I have shown the procedure as it applies to a single screw or similar item, nevertheless it will be understood that. a multiplicity of patterns may be used simultaneously if desired. For example, a number of patterns such as that shown in Figure 1 may be originally arranged side by side with the stripl 25 extending into all of the aligned slits in the heads of the patterns.

The wax article that is ultimately cast in the mold'of Figure 6 would then consist of a series of wax pieces which, if a common sprue has been used, will be joined together by a connecting piece of wax.

It will be observed that the metal strip 25 is itself employed in the mold of Figure 6. )This strip may thus be said to be a mold portion which is separately formed or constructed and which is adapted to fit snugly intofa recess in the original pattern, i. e., the slit 23. As another example of the use of such a specially-formed mold portion I have shown il. pattern 32 in Figure 7 in which there is atraniiverse recess 33 of more substantial depth.

In carrying out the present procedure with respect to the pattern 32, a separate mold portion 34 is first produced by any suitable means, the portion 34 being adapted to fit snugly. into the recess 33. It may extend for the full depth of the recess 33, or it may be shorter than this recess, depending upon requirements. In the ilthe recess 33, and the opposite ends of the 'pore tion 34 are slightly convexed.

The mold portion sa is met positioned within' The parts are then separated, the block 3l dis-` carded, the augmented pattern is laid into the mold section C, and the membrane 3l is again applied to the exposed portions ofthe augmented pattern and of the mold section C. The comg plementarymold l.section D is then cast, as' in' dicated in Figure 11.

Ultimately.I the mold portion is disassociated v from the pattern 32y and positioned into corresponding association with the complementary' `mold sections jC and D as shown in Figure y,l2-.r The original convexity imparted to the `ends of the portion 3l will have resulted in the formationy of corresponding depressions in the mold sec'- tions C and D by means of which the positioning parting from they spirit and scopeof theinven-' tion vas expressed `in thevappended'claims. It is 1 therefore intended thatvthese details` unlessuoth; i f

as illustrativel and fg i lustrated case, the mold portion 34 is causedto f embody a depth substantially equal to that of t notinalimitingsense; 1

vHaving thusuescribeq mir-'inventionmrdii-` lustrated its use,` what I claim as newy and '2 A medium consist-ing yofi: lfreely `stretchable?mism.;

of the mold tated. y

A suitable sprue or sprues 31 are provided, and the mold of Figure 12 is ready to be used in the lportion 34, as in Figure 12,- is facilicasting ofv articles, either of wax or any "other desired material.

It will be understood that the originel `pii'tfterns shown in the present drawings are merely illustrative, yand that the invention ,is by no means restricted to thevcreation of molds whose cavities have these particular shapes Moreover,-

while the original pattern is in each case described as being embedded in a supporting yblock of plaster or the like, the use of such plaster support is not in every case essential.. The parv ticular manner of applying the stretchable membrane successively to different selected portions of the original pattern, and successively casting complementary mold sections under pressure against the membrane, will depend in large meas ure upon the contours and nature of the mold.

cavity desired. In each case, however, the parting medium is a freely stretchable membrane oi' inappreclable thickness. ciable I intend to refer toa membrane that is as thin as it is possible to obtain, and preferablyv By the term inappreing element disclosed in United statesflsatent No. 1,834,123, the purpose of which is-notzonl'y. r l diametrically opposite from the purpose for which the present membrane is used, but whose thickness is `definitely and deliberately required to be of appreciable magnitude, even thoughslight;

In general, it will be understood that the -Yd-e-k' ,tails herein described and illustrated maybe modified by those skilled in the art without de# erwise specified be interpreted 'tosecure'byLettersPatentisz'yj i r 1. inthe art of making s iionow casting mold' whose castingv cavity has substantially", the.;same

configuration and volumetric -proportionsas af Y- predetermined pattern and rwhose inner? surface accurately 1 conforms to. the convolutions' f said pattern, the steps'whicli consist'zinapplying to aselected portion of the patterns-parting ybrane of inappreclable ,mold section against sn'mvlmeunnll l Such a portion of the pattern, the space between said mold section and pattern being thereby at no place of absolute' zero thickness nor of greater thickness than said membrane.

2. In the art'of making a hollow casting mold whose 'casting cavity has substantially the same configuration and volumetric proportions as a predetermined pattern and whose inner surface accuratelyV conforms to the convolutions of said pattern, the stem which consist in applying successively to selected different portions of the pattern a parting medium consisting of a freely stretchable membrane of inappreciable'thickness, and successively casting complementary mold sections against the membrane-covered portions of the pattern, the space between said mold sections and pattern being thereby at no place -oi absolute zero thickness nor of greater thickness than said membrane.

3. In the art of making a hollow casting mold whose casting cavity has substantially the same configuration and volumetric proportions as a predetermined pattern and whose inner surface accurately conforms to the convolutions of said pattern, the steps which consist in first applyascetic parting medium to the exposed portion of the pattern and mold section, and casting a complementary-mold section against said membranecovered surfaces, the space between the mold sections and the pattern being thereby at no place oi absolute zero thickness nor of greater thickness than said membrane.

4. In the art of making a hollow casting mold whose casting cavity has substantially 'the same conguration and volumetric proportions as a predetermined pattern having a recess therein and whose inner surface accurately conforms to the convolutions of said pattern, the steps which consist in tlrst forming a mold portion adapted to iit snugly into said recess, positioning said mold portion within said recess to form an augmented pattern, then applying successively to'l selected different portions of the augmented pattern a parting medium consisting ofa freely stretchable membrane of inappreciable thickness, successively casting complementary mold sections against the membrane-covered surfaces, the space between saidmold sections and augmented pattern being thereby at no place of absolute zero thickness nor of greater thickness than said membrane, and finally disassociating said mold portion from said pattern and positioning it into corresponding association with said complementary mold sections.

ERIC H. ZAHN. 

